This invention relates to sewer pipe structures. More particularly but not exclusively, the invention relates to combined sewer overflow structures. The invention also relates to their method of construction.
It is known to construct a combined sewer overflow structure in which sewage or rain water mixed with sewage can flow through an inlet pipe into a storage tank constructed of concrete. At the opposite end of the storage tank is a main outflow pipe continuously feeding either the sewage or mixture of water and sewage to a sewage treatment plant.
During dry weather, a continuous flow of sewage is possible through the storage tank to the sewage treatment plant, using the main outflow pipe only. In a rain storm condition, there will be a rush of water into the storage tank, and the main outflow pipe may not be able to cope with the mixture of water and sewage. The water level in the storage tank will rise, and different items of sewage will be separated out, solids tending to sink, and floatables rising with the water level. Previously, if required, excess water would be allowed to escape from the storage tank through an upper discharge outlet, e.g. into a river, but a proportion of the sewage would also tend to flow out, thus polluting the environment in a manner which would now be unacceptable.
A known improved method is to provide a weir/scum board arrangement in the storage tank, which acts as a stilling pond. Relatively clean water flows over the weir and is discharged through a spill outflow, whilst the floatables are retained within the storage tank by the scum board. A mixture of water and sewage continues to be discharged through the main outflow pipe to the sewage treatment plant. It is known to provide side weirs which extend longitudinally of the storage tank or an end weir which is transverse to the direction of flow through the structure.
In the case of side weirs their efficiency is partly dependent on their length, since the longer the weir, the shallower is the water flowing over the weir. Similarly the efficiency of an end weir is partly dependent on its distance from the inlet to the storage tank and the extent to which the speed of flow has declined, whereby a greater depth of overflow can be tolerated.
As the water level falls the floatables drop to the bottom of the storage tank and flow through the main outflow pipe to the sewage treatment plant.
Construction of the concrete storage tank with its weir/scum board arrangement is a complicated task involving a considerable amount of time and labour on site.
Therefore, there is a need to facilitate the construction of a modular, prefabricated storage tank, together with an inlet pipe and at least one outlet pipe, and any internal weir/scum board arrangement.
According to the invention there is provided a sewer pipe structure comprising a storage tank having at one end an inflow pipe of predetermined diameter and at the opposite end at least one outflow pipe whereby sewage alone or a mixture of water and sewage is permitted to flow through the structure, wherein the structure is a modular prefabricated pipe structure in which the storage tank is at least one pipe length, and at each end of the storage pipe there is provided an end cap formed of a pipe of the same nominal diameter as the storage pipe, with one closed end wall and the inflow pipe, in the case of the upstream end cap, and at least one outflow pipe, in the case of the downstream end cap, extending through the closed end wall of the respective end cap.
The storage pipe may be formed of a plurality of pipe lengths jointed together in series.
Preferably the storage pipe has a dry weather flow channel extending therethrough in an axial direction along its base, the channel interconnecting the inflow pipe in the upstream end cap and at least one outflow pipe in the downstream end cap.
The dry weather flow channel is preferably formed by two pipe sections extending longitudinally along the base of the storage pipe in spaced relation to one another. The pipe sections preferably define a generally V-shaped channel in cross-section narrowing towards the bottom of the channel, the convex surfaces of the two pipe sections facing each other.
Preferably the storage pipe, or at least one of the pipe lengths forming the storage pipe, includes an overflow weir allowing water flowing over the weir to flow to at least one spill outflow, preferably a pipe in the outer wall of the storage pipe, through which the overflow water is discharged from the pipe structure.
The overflow weir may be at least one side weir extending longitudinally of the storage pipe, and wherein means are provided for conveying the water which flows over the weir to the or each spill outflow. Preferably the or each side weir is formed by the lip of a channel formed by a pipe section attached to or suspended from the internal wall surface or the top of the storage pipe, for conveying the water which flows over the weir to pipework communicating with the or each spill outflow.
Preferably the or each side weir channel is more than a semicircle to give it high sides and is open at its top, along its length.
Alternatively, an end overflow weir is provided close to the downstream end of the storage pipe, the weir extending transversely to the length of the storage pipe for allowing an overflow of water to flow to a spill outflow through which the overflow water is discharged from the pipe structure.
Preferably the or each weir is provided with an associated scum board extending substantially parallel to the weir and downwardly to a level below the level of the weir, and which is positioned sufficiently close to the weir to resist floatables passing over the weir.
Preferably each pipe length and pipe section is formed of GRP.
The closed end wall of each end cap is preferably formed by an infill of concrete or a precast concrete panel.
The invention also provides a method of constructing a modular, prefabricated sewer pipe structure comprising forming one or more pipe lengths to be connected together in series to form a storage tank, forming an upstream end cap with one closed end wall and an inflow pipe of predetermined diameter, forming a downstream end cap with one closed end wall and at least one outflow pipe, and assembling the pipe lengths and end caps to form the pipe structure in which sewage alone or a mixture of water and sewage can flow through the structure.